I haven't played DS in a while, but this is something that came to my mind the other day.

First of all, I assume that things in Lordran didn't start going down right after the victory over the Everlasting Dragons and at the beginning of the Age of Fire, Gwyn&co.'s "golden age". And so, Gwyn had no idea that one day he would have to sacrifice himself into the First Flame to prolong his Age.And, unaware of this, he bequeathed shards of his soul to Seath and the Four Kings for their services.

Now, to enter the Kiln we must satiate the Lordvessel with the three Lord Souls. Getting Nito's and the Witch's ones is pretty "easy", we just need to bust into their homes and kill them. But we are pretty lucky that before entering the Kiln and shutting the doors behind him, Gwyn gave those shards to Seath and the Four Kings, otherwise noone would be able to enter and succeed him in his duty, lacking the last Lord Soul.

So, do you think Gwyn just got lucky to unconsciously leave behind him pieces of the key to the Kiln, certain that his strength was more than enough to keep the Flame alive indefinitely?

Or he actually did know that someday, someone would have to take his place and thus he had to leave part of his soul in Lordran to let this someone enter the Kiln? And if this is the case, did he really leave the future of his Age and of his world in the hands of Fraampt and Gwyndolin, hoping they could manipulate his possible successor into linking the Flame? (After all, we can follow Fraampt advices all game long, but after killing Gwyn we can do whatever we want, without regard to which snake we allied with)

Greatsword born from the soul of Gwyn, Lord of Cinder. As bearer of the ultimate soul, Gwyn wielded the bolts of the sun, but before linking the fire, divided that power amongst his children, and set off with only this great sword as his companion.

Soul of one of the Four Kings, who fell toDark. A fragment of a Lord Soul discoveredat the dawn of the Age of Fire.

Lord Gwyn recognized the foresight of thesefour great leaders of New Londo, and grantedthem their ranks and the fragments of agreat soul. Although this is not a full LordSoul, it can still satiate the Lordvessel.

Soul of the albino Seath the Scaleless.A fragment of a Lord Soul discoveredat the dawn of the Age of Fire.

Seath allied with Lord Gwyn and turned uponthe dragons, and for this he was awardedDukedom, embraced by the royalty, and givena fragment of a great soul. Although just apiece, it will still satiate the Lordvessel.

Soul of Gwyn, the Lord of Sunlight and Cinder, who linked the First Flame.

Lord Gwyn bequeathed most of his power to the Gods, and burned as cinder for the First Flame, but even so, Lord Gwyn's soul is a powerful thing indeed.

Lordvessel bestowed upon the chosen Undead who is destined to succeed Lord Gwyn. The chosen Undead is granted the art of warping between bonfires. To open the final door, place this vessel on the Firelink Altar, and fill it with powerful souls.

Frampt's Dialogue

As Kingseeker, I shall now instruct you, the Lord's successor in your next task. To achieve your fate, fill the vessel with powerful souls, commensurate to the great soul of Gwyn. Scarce few possess such brilliant souls. Gravelord Nito, the Witch of Izalith, the Four Kings of New Londo, who inherited the shards of Gwyn's soul…and Lord Gwyn's former confidant, Seath the Scaleless. All of their souls are required to satiate the lordvessel

Comensutate definition.

commensurate

.

adj. adjective

1. Of the same size, extent, or duration as another.

2. Corresponding in size or degree; proportionate

With all that quoted I will now answer your question.

The Bequeathed souls were a small fragment of Gwyn's power and were gifted to Seathe and the 4kings. He never seemed to plant to have them used to open the Kiln.

When Gwyn did leave for the Kiln he sought to preserve most of his power in his children. So he gave all except the amount of power he needed to opnen the Kiln, to Gwynevere and Gwyndolin.

Gwyn was the most powerful Lord. Often he ws stated to have the Ultimate soul.

Frampt and Kaathe seek to have someone either Succeed Gwyn or overthrow him. In order to do so he must be killed. Gwyn locked himself in the Kiln to prevent anyone from interfering with relinking the flame. He did not expect to Hollow and most likely expected to return and retrieve his power from his children. This did not happen.

The serpents realized that someone with equal power to Gwyn was needed to unlock the Kiln. The quickest way for someone to become as powerful was by retrieving the gifted fragments and replacing the pieces he gave to his children. By Frampt more than likely working with Gwyndolin and Gwynovere being an illusion, it was impossible to reclaim those two fragments. Surrogates were needed and the Witch of Izalith and Gracelord Nito were the perfect candidates.